1. Field of the Invention
This invention relates to sound-generating devices, and more particularly, to flexible piezoelectric loudspeakers.
2. Background of the Invention
In the recent years, there have been continued developments for electronic products. One design concept has been providing lightweight, thin, portable and/or small devices. In this regard, flexible electronic technology has been increasingly used in various applications, such as thin-screen displays, LCDs, flexible circuits and flexible solar cells. Applications for flexible electronics, such as flexible speakers, may benefit from their low profile, reduced weight, and/or low manufacturing cost.
A loudspeaker may produce sound by converting electrical signals from an audio signal source into mechanical motions. Moving-coil speakers are widely used currently, which may produce sound from the back-and-forth motion of a cone that is attached to a coil of wire suspended in or movably coupled with a magnetic field. A current flowing through the coil may induce a varying magnetic field around the coil. The interaction of the two magnetic fields causes relative movements of the coil, thereby moving the cone back and forth. This compresses and decompresses the air, and thus generating sound waves. Due to structural limitations, moving-coil speakers are less likely to be made flexible or in a low profile.
Flexible piezoelectric loudspeakers, such as piezoelectric polyvinylidene fluoride speakers, may be made of flexible polymer materials. With electric polarization, the flexible polymer material may possess characteristics of permanent polarization and resistance to environmental conditions. Thus, such flexible polymers are suitable for being fabricated as loudspeakers.
U.S. Pat. No. 4,638,207 relates to a piezoelectric balloon speaker with a piezoelectric polymer film. The inflated balloon may provide tension for the piezoelectric polymer film. In addition, the resonance frequency may be adjustable by the pressure applied to the balloon. However, such a speaker may not be fabricated as a low-profile flexible loudspeaker. U.S. Pat. No. 6,504,289 relates to a piezoelectric transducer for transmitting acoustic energy. The transducer is enclosed in a rigid enclosure and thus cannot be made flexible. U.S. Pat. No. 6,349,141 relates to a flexible audio transducer with a balloon structure. The balloon structure may result in some issues on structure strength and designs relating to resonance frequency. U.S. Pat. No. 6,717,337 relates to an acoustic actuator with a piezoelectric drive element made of piezoelectric ceramics in the lead zirconate titanate (PZT) or PZT derivatives. In response to the radial contraction and expansion of the piezoelectric drive element, an acoustic diaphragm may vibrate to generate sound waves. The piezoelectric ceramics however are vulnerable and susceptible to fragmentation.